Torque wrenches of the class here concerned with are provided to apply measured predetermined torque or bias forces onto and through various screw fastening means, such as headed screw fasteners engaged in work pieces. Such wrenches characteristically include elongate lever arms with fastener engaging means or points of engagement at one of their ends and manually engageable handles at their other ends. Further, the class of torque wrenches here concerned with is further characterized by the inclusion or provision of elongate, elastic, deflection beam portions or sections extending longitudinally of the lever arms, between the ends thereof and which bend under the work forces applied to and conducted through the wrenches and by means which measure the deflection of the beams and which translate and display the measured deflection of the beams, by means of indicating instruments, in suitable force units.
In most torque wrenches of the class here concerned with, the means for measuring deflection of the deflection beams and the indicating instruments have been mechanical means and instruments.
The number of such wrenches incuding electrical or electronic means for measuring deflection of the deflection beams and related electrical or electronic indicating instrument, such as my invention includes, is limited.
The most pertinent prior art that I am aware of is that torque wrench which is described in U.S. Pat. No. 4,006,629. That patented wrench structure has or is provided with four strain gauges attached to the deflection beam, there being two strain gauges attached to one side and two strain gauges attached to the opposite or other side of the beam. The pairs of strain gauges at the opposite sides of the deflection beam are spaced differently longitudinally of the beam and from the fastener engaging means or point of engagement of the wrench structure. The four strain gauges establish the four resistors of a Wheatstone bridge, the output voltage of which is representative of the deflection of the beam and of the torsional force applied by the wrench onto a related fastener or the like. As long as there is no load or force applied through the wrench, the output voltage of the bridge is zero. Changes in the length of the beam resulting from thermal expansion and contraction act symmetrically on the four gauges, precluding a change in the voltage output of the bridge. When the wrench is used and the beam bends, the pair of strain gauges at one side of the beam are stretched or lengthened and the pair of gauges at the other or opposite side of the beam are compressed or shortened. Lengthening and shortening of the gauges in the above manner changes their resistance and an output voltage of the bridge is obtained which deviates from zero. That output voltagg is supplied to an indicating instrument for digital display of the measured value by means of a corresponding converter.
The foregoing prior art wrench requires a constant power supply to the bridge, converter and indicating instrument whereby power consumption is so high that an outside power supply must be provided. A self contained high capacity battery power supply capable of sustaining operation of the noted wrench structure would be excessively large and heavy for practical use of that wrench structure.
A further disadvantage of the noted prior art wrench resides in the fact that balancing and adjustment of the wrench is not possible subsequent to attachment of the strain gauges to the deflection beam and the finished dimensions of the beam and of the gauges involve tolerances. Also, attachment and positioning of the gauges on the beam involves tolerances. Deviations in the wrench structure due to such tolerances are cumulative and cause errors in measurement which cannot be balanced or otherwise compensated for. Still further, the finished dimensions of the strain gauges (involving tolerances) have associated therewith the proportionality factor K indicative of the ratio of change in length to corresponding change in resistance.